The Global System for Mobile Communication (GSM) is one of the most widely deployed communication standards for mobile wireless communication. In order to introduce packet-switched technology, general packet radio service (GPRS) was developed by the European Telecommunications Standards Institute (ETSI). One limitation of GPRS is that it does not support voice services. Other issues with GPRS include lack of higher data rates supported as well as poor link adaptation algorithms. Therefore, the third generation partnership project (3GPP) developed a new standard for GSM to support high rate data services, released in 1999 and known as enhanced data rates for GSM evolution (EDGE).
A network configured according to these standards comprises a core network (CN), at least one wireless transmit/receive units (WTRU) attached to a radio access network (RAN), such as the GSM/EDGE radio access network (GERAN). The GERAN comprises a plurality of base transceiver stations (BTSs), each connected to and controlled by a base station controller (BSC). The combination of the BSCs and the corresponding BTSs is realized as the Base Station System (BSS).
The radio link control/medium access control (RLC/MAC) protocol, which resides in the WTRU and the BSS, is responsible for reliable transmission of information between the WTRU and the network. In addition, the physical layer latency, (for example, packet transfer and serialization delays) is controlled by the RLC/MAC protocol.
A goal for GERAN evolution is to develop new technology, new architecture and new methods for settings and configurations in wireless communication systems. One work item for GERAN evolution is latency reduction. Release 7 (R7) of the 3GPP GERAN standard introduces several features that may improve throughput and reduce latency of transmissions in the uplink (UL) and the downlink (DL). UL improvements are referred to as higher uplink performance for GERAN evolution (HUGE), and DL improvements are referred to as reduced symbol duration higher order modulation and turbo coding (REDHOT). Both of these improvements may generally be referred to as evolved general packet radio service 2 (EGPRS-2) features.
The Latency Reduction feature includes two (2) technical approaches that may operate either in a stand-alone mode, or in conjunction with any of the other GERAN R7 improvements. One approach uses a fast positive acknowledgement/negative acknowledgement (ACK/NACK) reporting (FANR) mode. Another approach uses a reduced transmission time interval (RTTI) mode. A WTRU may operate in both FANR and RTTI modes of operation with legacy EGPRS modulation and coding schemes (MCSs), and with the newer EGPRS-2 modulation and coding schemes.
REDHOT and HUGE provide increased data rates and throughput compared to legacy EGPRS DL and UL. These modes may be implemented through the use of higher order modulation schemes, such as sixteen quadrature amplitude modulation (16-QAM) and thirty two quadrature amplitude modulation (32-QAM). These modes may also involve the use of higher symbol rate transmissions and turbo-coding. Similar to legacy systems, REDHOT and HUGE involve an extended set of modulation and coding schemes that define new modified information formats in the bursts, various coding rates and coding techniques and the like.
Prior to the introduction of FANR, ACK/NACK information was typically sent in an explicit message, referred to as an RLC/MAC control block, which contained a starting sequence number and a bitmap representing radio blocks. The reporting strategy (how and when reports are sent, and the like) was controlled by the network. The WTRU would send a Control Block as a response to a poll from the base station system (BSS). The poll will also include information about the UL transmission time (for example, when the WTRU is allowed to send its control block in the UL). During normal operation, when higher layer information is exchanged between the WTRU and the network, the information transfer occurs using RLC Data Blocks.
A drawback of the current ACK/NACK reporting protocols is that a full control block is needed every time ACK/NACK information is sent. Therefore, a large overhead is required when ACK/NACK information is frequently needed for delay sensitive services.
Consequently, within the framework of GERAN evolution, a new ACK/NACK state machine that uses ACK/NACK reports “piggybacked” on RLC Data Blocks in the opposite link direction was introduced.
This protocol has the potential to significantly reduce the retransmission delay without significant overhead. These piggybacked ACK/NACK (PAN) reports are bitmaps, designed as a combination of block sequence numbers (BSNs) which specify outstanding radio blocks bitmaps giving ACK/NACK information of radio blocks, and size bits or extension bits specifying the size of the ACK/NACK information. PANs are used to transmit an ACK/NACK bitmap within a radio block carrying RLC data.
This allows for ACK/NACK information to consist either of one single PAN or to be split into several multiple segment PANs. This allows for a decrease in latency and round-trip times due to increased flexibility of sending ACK/NACK reports independently from data transmissions to a particular wireless transmit/receive unit (WTRU) without necessitating special RLC/MAC control blocks, while maintaining general principles of RLC window operation.
FIG. 1 shows a conventional radio block. Currently, a PAN field may be inserted into a RLC/MAC radio block using modulation and coding schemes (MCSs) for EGPRS or new MCSs provided by REDHOT/HUGE (EGPRS-2). In both of these scenarios, the radio block consists of a separately encoded RLC/MAC header 105 that is decodable independent from the RLC data payload; an RLC data payload 110 and a PAN field 115 that is separately decodable from the RLC/MAC header and RLC data payload.
Some legacy EGPRS radio blocks and some new REDHOT/HUGE radio blocks may contain more than one RLC data Protocol Data Unit (PDU) per radio block. The PAN is mapped on the burst together with the data. The placement of the PAN before interleaving is dependent on the interleaving depth of the data block. Since all PANs have low code rates, a maximized interleaving depth is preferred.
The insertion of the PAN field 115 into the radio block requires heavier puncturing of the actual RLC data payload. In essence, since the overall number of bits that may be placed into the radio block is fixed, more encoded data bits must be removed from the RLC data payload once a PAN is inserted. Since the RLC/MAC header coding remains unchanged even when a PAN is inserted, the coding rate of the data portion should be increased. However, this may be detrimental to link performance and effectiveness of the link adaptation algorithm, because the increased channel coding rate and reduced number of channel bits of the affected RLC data payload 110 of the radio block may lead to more transmission errors and less protection of the data.
Another problem is that the RLC/MAC header 105, the RLC data payload 110 and the PAN field 115 are all independently channel coded. For example, a PAN field, which contains M=20 information bits and N=6 cyclic redundancy check (CRC) bits, is coded into 80 channel coded bits yielding a coding rate of approximately 0.33. Therefore, balancing error performance of the RLC/MAC header 105, the RLC data payload 110 and the PAN field 115 is essential to good performance of the radio block.
The different error performances of the portions making up the RLC MAC radio block 110 are shown in FIG. 2. For example, if the error rate of the RLC/MAC header 105 becomes too high, more transmissions are lost due to the receiver (WTRU or base station) failing to decode the RLC/MAC header 105, rather than errors in the RLC data payload 110. The protection of the PAN field 115 is also questionable, as well as the mapping of the PAN field 115.
In the conventional RLC/MAC radio block of FIG. 1, the RLC/MAC header 105, the RLC data payload 110 and the PAN field 115 are interleaved together. Their channel-coded bits carried by the modulation symbols are spread across four (4) radio bursts such that bits belonging to the PAN field 115, for example, are not necessarily contiguous. Applying a power offset just to a subset of PAN-carrying symbols may create extra leaking of transmit (Tx) power into the adjacent carriers due to radio frequency (RF) non-linearity from “normal” symbols transiting to symbols sent at higher offset power at the configured standard peak-to-average ratio (PAR) back-off for the given modulation order. This may result in intolerable out-of-band emission levels.
It is therefore desirable to have a method and apparatus for linking performance and error resilience of different portions of a radio block and matching portions of a radio block to their respective requirements for PAN filed inclusion, when compared to transmission without PAN field inclusion, without changing the number of channel coded bits.